Correlation Of Colloidal Characteristics With Exfoliation Of Montmorillonite And Preparation Of Si Nanosheets

The lamellar structure and hydration expansion properties of montmorillonite make it easy to delaminate and exfoliate into two-dimensional nanosheets in aqueous solution under external force.This phenomenon is widely existing and used in many fields.The studies of the preparation methods,colloidal characteristics of montmorillonite nanosheets and the preparation of Si nanosheets using montmorillonite nanosheets as precursors were conducted in the thesis,which is of great significance to explore the exfoliation mechanism of montmorillonite,establish the relationship between the exfoliation of montmorillonite and colloidal characteristics,expand the application fields of montmorillonite and enhanc the utilization value of montmorillonite resources.In this paper,montmorillonite nanosheets were prepared via ultrasonic exfoliation and freezing-thawing exfoliation method from sodium montmorillonite.The effects of ultrasonic power,freezing time and cycle indexes of freezing/thawing on the thickness and lateral size of montmorillonite nanosheets were detailedly studied.Through turbidity measurement and theoretical analysis,the relationship between turbidity and exfoliation of montmorillonite was studied.The influencing factors of turbidity method for characterizing the exfoliation degree of montmorillonite were studied.The effect of surface electrical property on the exfoliation of montmorillonite and the preparation and colloidal characteristics of montmorillonite nanosheets with different diameter-thickness ratios were studied.The Si nanosheet for lithium-ion battery anode was prepared by magnesium thermal reduction method using the exfoliated montmorillonite nanosheets as the precursor.The effects of stacking methods of montmorillonite nanosheet on the electrochemical properties of Si nanosheet were studied.The main conclusions are displayed as follows:?1?The montmorillonite nanosheets were prepared from the new freezing-thawing exfoliation method.The comparison of ultrasonic exfoliation and freezing-thawing exfoliation method was also studied.The ultrasonic method has better exfoliation efficiency but tends to damage the nanosheets.;while freezing/thawing method can exfoliate montmorillonite tactoids with low efficiency while virtually retaining the original lateral size.?2?Turbidity method can used to characterize the exfoliation degree of montmorillonite according to the linearly relationship bwtween the turbidity ratio of montmorillonite suspension before and after exfoliation with exfoliation degree.This method should be applied between pH value 4–10 and below the solid concentration of2%w/v.?3?The layer charge density of montmorillonite can significantly influence the exfoliation degree of montmorillonite platelets.The degree of exfoliation of MMT decreased as the decrease of the layer charge density.And the negative mobility increased with the increase of exfoliation degree.?4?By combining them and reasonably controlling the cycle index of freezing/thawing and ultrasonic power,the montmorillonite nanosheets with different aspect ratios have been prepared efficiently.And the negative mobility and stability of suspension of montmorillonite nanosheets increased with the increase of aspect ratios.?5?Montmorillonite nanosheets were used as precursors to synthesis the Si nanosheets via Mg thermal reduction.It was found that the Si nanosheets prepared by stacking montmorillonite nanosheets with card-house-like structure in the presence of NaCl protectant had good crystal structure,complete sheet and abundant pore size and the better electrochemical performance,exhibits a high reversible capacity of 1815mAh g^-1 at a current density of 0.5 A g^-1 after 100 cycles.